1. Field of the Invention
The present general inventive concept relates to a method of forming a thick layer more uniformly by screen printing, and a method of forming a piezoelectric actuator of an inkjet head using the method of forming the thick layer.
2. Description of the Related Art
Inkjet heads are devices for printing an image on a printing medium by ejecting ink droplets onto a desired region of the printing medium. Depending on an ink ejecting method, the inkjet heads can be classified into two types: thermal inkjet heads and piezoelectric inkjet heads. A thermal inkjet head generates bubbles in an ink to be ejected by using heat and ejects the ink utilizing an expansion of the bubbles, and a piezoelectric inkjet head ejects an ink using a pressure generated by deforming a piezoelectric material.
FIG. 1A is a sectional view illustrating a conventional piezoelectric inkjet head, and FIG. 1B illustrates a sectional view taken along line A-A′ of FIG. 1A. The conventional piezoelectric inkjet head illustrated in FIGS. 1A and 1B is formed by a conventional screen printing method.
Referring to FIGS. 1A and 1B, a manifold 11, a plurality of restrictors 12, and a plurality of pressure chambers 13 forming an ink flow channel are formed in a flow channel plate 10 of the conventional piezoelectric inkjet head. A vibration plate 20, which can be deformed by piezoelectric actuators 40, is bonded to a top surface of the flow channel plate 10, and a nozzle plate 30 in which a plurality of nozzles 31 are formed is bonded to a bottom surface of the flow channel plate 10. The vibration plate 20 can be formed integrally with the flow channel plate 10, and the nozzle plate 30 can also be formed integrally with the flow channel plate 10.
The manifold 11 is an ink passage for supplying ink from an ink reservoir (not illustrated) to the respective pressure chambers 13, and the restrictors 12 are ink passages allowing inflow of ink from the manifold 11 to the pressure chambers 13. The pressure chambers 13 are filled with and eject the supplied ink and are arranged at one side or both sides of the manifold 11. The nozzles 31 are formed through the nozzle plate 30 and are connected to the respective pressure chambers 13. The vibration plate 20 is bonded to the top surface of the flow channel plate 10 to cover the pressure chambers 13. The vibration plate 20 is deformed by the operation of the piezoelectric actuators 40. Thus, pressures in the respective pressure chambers 13 change and the ink is ejected from the pressure chambers 13 by the operation of the piezoelectric actuators 40. Each of the piezoelectric actuators 40 includes a lower electrode 41, a piezoelectric layer 42, and an upper electrode 43 that are sequentially stacked on the vibration plate 20. The lower electrode 41 is formed on the entire surface of the vibration plate 20 as a common electrode. The piezoelectric layer 42 is formed on the lower electrode 41 above each of the pressure chambers 13. The upper electrode 43 is formed on the piezoelectric layer 42 as a driving electrode for applying a voltage to the piezoelectric layer 42.
In the conventional piezoelectric inkjet head, the piezoelectric actuator 40 is usually formed as follows. The lower electrode 41 is formed by sputtering a metal to a predetermined thickness on a top surface of the vibration plate 20. The piezoelectric layer 42 is formed by applying a piezoelectric ceramic material paste to a predetermined thickness to a top surface of the lower electrode 41 through screen printing, and by sintering the applied piezoelectric ceramic material paste. The upper electrode 43 is formed by applying a conductive material to a top surface of the piezoelectric layer 42 by screen printing and sintering the applied conductive material.
When forming the piezoelectric layer 42, the piezoelectric ceramic material paste is applied to the lower electrode 41 to a thickness of several tens of micrometers, and then the piezoelectric ceramic material paste is dried and sintered to obtain a thick layer for the piezoelectric layer 42. However, since the piezoelectric ceramic material paste applied to the lower electrode 41 is thick, the piezoelectric ceramic material paste spreads outward with time. This makes the piezoelectric layer 42 relatively thicker at a center portion and thinner at edge portions as illustrated in FIG. 1B, such that a thickness and a width of the piezoelectric layer 42 are not uniform. Further, an outer edge of the piezoelectric layer 42 can be curved although a straight outer edge is preferable. Accordingly, the overall shape of the piezoelectric layer 42 can be irregularly curved.
Furthermore, due to the unevenness of the piezoelectric layer 42, the thickness and the width of the upper electrode 43 formed on the piezoelectric layer 42 may not be uniform. In addition, the distance between the lower electrode 41 and the upper electrode 43 is not constant because of the uneven thickness of the piezoelectric layer 42, and thus an electric field cannot be uniformly formed between the lower electrode 41 and the upper electrode 43.
Additionally, nozzle density should be high to realize high-quality printing such as high-resolution and high-speed printing. The nozzle density is usually denoted using “cpi (channel per inch),” and the resolution of an image is usually denoted using “dpi (dot per inch).” To increase the nozzle density, the distance between adjacent pressure chambers 13 should be reduced. Accordingly, the distance between adjacent piezoelectric layers 42 should be reduced. However, as described above, since the piezoelectric layer 42 formed by a conventional method has an uneven width, the piezoelectric layer 42 may easily make contact with an adjacent piezoelectric layer 42 when they are formed closer to each other, making it difficult to increase the nozzle density much more.
As mentioned above, in the conventional method of forming a thick layer by screen printing, the thickness and width of the piezoelectric layer 42 of the piezoelectric actuator 40 cannot be uniformly formed. Further, the conventional method makes it difficult to increase the nozzle density of the inkjet head.